To begin with, neither of them ever really managed to realize that the children weren't selfish, greedy, malicious freeloaders who spent their days plotting the overthrow of the parents they clearly hated. This is owing to their confusing disagreement with defiance and their taking themselves way too damned seriously.
They also don't know who the people in their children's lives really are. They don't see Anthony for being the horrible person he is, they don't see that Warren or Paul would be better choices because their lifestyle scares them and they can't see that Thérèse is a horribly ill-used woman lashing out because she's being beset on all sides by clannish vermin working to curry favor with two greedy boomer scuzzbags who think the world owes them a living.
Finally, this inability to know who people actually are is why they got married and why they stayed married. John and Elly insist on seeing one another as any number of things that they are not and refuse to see the truth lest they be confronted with the terrifying realization that they're a couple of goobers.